Remove Element

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27.Remove Element

Given an integer array nums and an integer val, remove all occurrences of val in nums in-place. The relative order of the elements may be changed.

Since it is impossible to change the length of the array in some languages, you must instead have the result be placed in the first part of the array nums. More formally, if there are k elements after removing the duplicates, then the first k elements of nums should hold the final result. It does not matter what you leave beyond the first k elements.

Return k after placing the final result in the first k slots of nums.

Do not allocate extra space for another array. You must do this by modifying the input array in-place with O(1) extra memory.

Example 1:

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Input: nums = [3,2,2,3], val = 3
Output: 2, nums = [2,2,_,_]
Explanation: Your function should return k = 2, with the first two elements of nums being 2.
It does not matter what you leave beyond the returned k (hence they are underscores).

Example 2:

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Input: nums = [0,1,2,2,3,0,4,2], val = 2
Output: 5, nums = [0,1,4,0,3,_,_,_]
Explanation: Your function should return k = 5, with the first five elements of nums containing 0, 0, 1, 3, and 4.
Note that the five elements can be returned in any order.
It does not matter what you leave beyond the returned k (hence they are underscores).

个人解答：

时间复杂度：O(n^2)

空间复杂度：O(1)

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int removeElement(vector<int>& nums, int val) {
    vector<int>::iterator pos = find(nums.begin(), nums.end(), val);
    while (pos != nums.end()) {
        nums.erase(pos);
        pos = find(nums.begin(), nums.end(), val);
    }
    return nums.size();
}

双指针：

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int removeElement(vector<int>& nums, int val) {
    int left = 0, right = nums.size();
    while (left < right) {
        if (nums[left] == val) {
            nums[left] = nums[right - 1];
            right--;
        }
        else {
            left++;//直到right指针指向数据不等于val,left左移
        }
    }
    return left;
}